Deductions impress clients, but incorrect deductions can be disastrous. Deductive reasoning has its place in therapy, but only as a means of generating hypotheses on which you can work with clients.

A client who usually had bare arms arrived in a long-sleeved shirt on a hot day. When I noted the change, she showed me cuts on her wrists which had become infected. I asked her to consider getting the wounds treated and, at the end of the session, said how glad I was she’d decided to do so. She asked how I knew she’d made that decision and I pointed out that she’d rolled up her sleeves. She was impressed by my deductive abilities: I felt like Sherlock Holmes. I was an idiot.

Once her scars were revealed, my client no longer had anything to hide from me, but she could have rolled down her sleeves again just as soon as she left the room. By jumping to conclusions under the guise of “deduction”, I could have damaged our rapport and possibly driven her away from seeking further assistance.

By a man’s finger nails, by his coat-sleeve, by his boot, by his trouser knees, by the callosities of his forefinger and thumb, by his expression, by his shirt cuffs — by each of these things a man’s calling is plainly revealed. That all united should fail to enlighten the competent enquirer in any case is almost inconceivable. (A Study in Scarlet, Sir Arthur Conan Doyle)

The ability of Sherlock Holmes to deduce all manner of information by simple observation has been popularised (and caricatured) for more than a century. The creator of Sherlock Holmes was trained in the same observational tradition that produced Sigmund Freud. Freud’s precise observations are admired even by those who reject his theories. The temptation to engage in Holmesian (or Freudian) deduction can prove hazardous for therapists (and their clients).

I have often read assessment letters wherein conclusions about beliefs and motivation are drawn from the client’s appearance and manner. These deductions have not been confirmed by—or even discussed with—the client but will, at best, colour and, at worst, direct subsequent interventions, again possibly losing the client (whether figuratively or literally, if assessing suicidality)

Deductions can and should be drawn from careful observation and questioning. These deductions should be treated as hypotheses, to be tested and confirmed (or ruled out) through further discussion and exploration with the client. Sherlock Holmes was always correct in his deductions. He had the advantage of being a fictional character, but even he observed proper caution.

I have devised seven separate explanations, each of which would cover the facts as far as we know them. But which of these is correct can only be determined by the fresh information which we shall no doubt find waiting for us. (the Adventure of the Copper Beeches, Sir Arthur Conan Doyle)

One of the inspirations for Sherlock Holmes is believed to be Dr Joseph Bell, with whom Sir Arthur Conan Doyle studied medicine. Dr Bell was renowned for his amazing deductions, but his aim was not to impress his clients: he was aiming to engage the enthusiasm of his students for less glamorous, but ultimately more helpful, patterns of work.

In teaching the treatment of disease and accident, all careful teachers have first to show the student how to recognize accurately the case.  The recognition depends in great measure on the accurate and rapid appreciation of small points in which the diseased differs from the healthy state.  In fact, the student must be taught to observe.  To interest him in this kind of work we teachers find it useful to show the student how much a trained use of the observation can discover in ordinary matters such as the previous history, nationality and occupation of a patient. (Dr Joseph Bell).